Many musical instruments utilize selective opening and closure of numerous openings in the instrument to change the sound produced by the instrument. Flutes, for example, typically have seventeen or eighteen pads which cover respective tone holes. Accurate and repeatable sealing of the tone holes is essential for producing crisp, clean tones.
The most prevalent pads are a composite of cardboard, wool felt, and a collagen membrane coveting the felt and attached to the cardboard. The pad is secured within a pad cup by a screw and washer or an adhesive. However, a great deal of effort is required to properly align the pad with the rim of the opening over which the pad is positioned. Highly skilled padders utilize paper or plastic shims of about 0.001 to 0.012 inch thick to adjust the alignment of the pad. Alternatively, certain pads can be floated into position on a bed of shellac or other adhesive. Each shimming adjustment requires the removal of the pad from the pad cup to position new shims. Proper padding of the flute often takes several hours to accomplish.
The larger the pad, the more difficult it is to establish a complete seal over the tone hole rim, as the error in pad position becomes magnified as the distance from the center of the pad increases. Because of this, larger pads (usually those 0.600 inch or greater in diameter) are normally held in position in a cup by mechanical means, and then are paper shimmed to adjust their conformity to the tone hole rim.
Various techniques have been developed to accurately and quickly position pads so that they seal over the tone hole rims. However, nearly all these techniques require that the pad be held in position in the flute cup by mechanical means in order to test and shim it. In one approach the pad is momentarily floated and must not be secured mechanically in the cup until after the pad is correctly shimmed. Unfortunately, the mere mechanical securing of the pad in the cup following accurate shimming can alter the relationship of the pad to the tone hole rim and disturb the sealing position of the pad.
In addition, the mechanical securing device contacts the pad on its compliant surface. This compliant surface is normally comprised of wool felt covered by animal membrane. The combination of a compliant pad surface with variability in the amount of torque used on the screw, the position of the washer, the angle of the bushing or other variables can vary the sealing capability of a pad even though the pad itself and the shimming materials behind it remain unchanged. Further, the compression caused in the pad by the mechanical securing device can also cause wrinkles in the membrane coveting the pad which are undesirable from an aesthetic and sealing standpoint.
In those pads which are normally mechanically secured to the cup the pad washer doubles as both a holding device and a resonator. In its function as resonator ,the washer is designed to deflect sound waves back into the flute. The hard surface of the resonator compared to the relatively soft surface of the pad reduces the damping properties of the pad, thereby decreasing energy loss.